A multitude of devices and methods for data communication over a network system are known. Network systems can be characterized by a number of design aspects, each typically having particular advantages and disadvantages. One design aspect of a network system is known as the "network topology." Examples of topologies which are in use are a bus topology, a ring topology, and a star or hub topology. In a bus topology, a single linearly configured communications line is provided, and each data terminal or node connects to the line at some point along its length. In a ring topology, each node or data terminal communicates directly with two neighbors arranged in a topologically circular manner. In a star topology, a central unit or hub communicates to each node along dedicated lines.
Another aspect of network design involves the data transfer rate or bandwidth type of a communication system. The two basic bandwidth types are broadband systems and baseband systems. In a broadband system, several data terminals share a single communication medium through a frequency-division scheme. In a baseband configuration, the signals are unmodulated, and sharing of a medium requires time division between the nodes. In either system, when more than one data terminal is configured for transmission along a single medium at the same time, neither data terminal can have exclusive use of the medium over the entire theoretically available bandwidth for that medium. Thus, the effective bandwidth, i.e., the data transfer rate which actually results between two nodes, becomes reduced when multiple nodes or data terminals, each configured for communication, are connected to a single medium.
A particular network system typically is designed for particular types of network topology and bandwidth type. For this reason, it is normally not possible to provide a data terminal and associated software which is configured, for example, for a bus topology and, without modification, construct such terminals in a ring or star topology. Similarly, it is normally not possible to connect, without modification, a data terminal configured for baseband communication to a broadband network.
A number of network systems are susceptible to the above-noted problem, wherein addition of nodes to the system causes an effective decrease in the bandwidth for each node. One particular such network system is the LocalTalk.TM. (formerly known as "Appletalk.RTM. Personal Network"). LocalTalk.TM. and Appletalk.RTM. are trademarks of Apple Computers, Inc., Cupertino, Calif. The LocalTalk.TM. system is a baseband network that uses a bus topology. The LocalTalk.TM. system is described as having a maximum or upper-limit bandwidth in its present configuration. In present descriptions of this product, the maximum bandwidth or raw data rate is stated to be 230.4 kilobits per second (Kbps). Of course, it is possible that future versions of LocalTalk.TM. will have a higher maximum bandwidth. In the present system, a single node on such a network could communicate at about 230 Kbps, if no other nodes were using the system for communication. A system with multiple nodes, each of which is attempting to make communication over the medium, results in an effective bandwidth for each node which is less than 230 Kbps. The amount of reduction of bandwidth depends on the number of nodes added and the amount of network activity attempted by each node.
The LocalTalk.TM. network system includes a number of well-defined characteristics. Access to the bus is managed by a protocol known as "Carrier Sense Multiple Access With Collision Avoidance" (CSMA/CA). The LocalTalk.TM. system also includes protocols for assigning node identifiers, addressing particular nodes, assembling and appending data transmission signals, such as frames, error detection, and the like. Because of the specificity of such protocols, as well as the specificity of the associated hardware and software, it is not possible to connect a LocalTalk.TM. network or node to another type of network without extensive modification. A more complete description of LocalTalk.TM. is found in Inside Appletalk, by Gursharan S. Sidhu, Richard F. Andrews, and Alan B. Oppenheimer, Apple Computer, Jul. 14, 1986, incorporated herein by reference to the extent needed for understanding the invention.
Interfacing devices, generally known as "gateways," have been developed for connecting a LocalTalk.TM. network to another type of network. These gateways, however, do not solve the problem of decreasing bandwidth resulting from increasing nodes, and thus merely pass on this problem to another network.
In spite of these difficulties, the LocalTalk.TM. network remains a heavily implemented system, and many installations are in existence which are configured to use or to be connected to such a system. Additionally, many types of software have been written for the purpose of using a LocalTalk.TM. network.
Accordingly, it would be useful to provide a network system which can accommodate existing hardware and software intended for previous decreasing-bandwidth systems, but which solves the problem of decreasing-bandwidth, i.e., which permits each node to communicate over the medium at the full medium bandwidth.